The invention relates generally to rotating electrical machinery and more specifically to slip rings for rotating electrical machinery.
Slip rings are employed in a wide range of rotating electrical machinery, such as electric motors and electric generators. A typical alternating current (AC) generator, for example, consists of a stationary stator and a rotor mounted within the stator. The stator includes a specific number of coils, each with a specific number of windings. Similarly, the rotor includes of a specific number of field poles, each with a specific number of windings. In addition to the rotor and stator, a generator has a collector assembly, usually including collector slip rings, brushes and brush holders. The rotor contains magnetic fields that are established and fed by an exciter. When the rotor is rotated, AC is induced in the stator. The changing polarity of the rotor, with respect to the stator, produces the alternating characteristics of the current. The generated voltage is proportional to the strength of the magnetic field, the number of stator coils and the number of windings in each coil, and the speed at which the rotor turns. Slip rings are electrical connections that are used to transfer power to and from the rotor of the AC generator. The slip ring may include a circular conducting material connected to the rotor windings and insulated from the rotor shaft. Brushes ride on the slip ring as the rotor rotates. The brush holder maintains the brushes in place, against the slip ring as the rotor rotates. The exciter supplies the DC current to the magnetic field of the rotor through one set of brushes and slip rings and completes a return path from another set of brushes and slip rings.
A wind turbine generator is a rotating electrical machine that is mechanically coupled to wind turbine blades. The wind turbine generator is positioned in a nacelle at the top of a wind turbine tower. The mechanical energy of the turbine is converted to electrical energy and delivered to a power grid through a collector system. An electronic power converter is used to control the flow of real and reactive power. One common configuration for the wind turbine generator is a doubly fed induction generator with a wound rotor and slip rings. More complex slip ring arrangements are required to convey three-phase ac power to the rotor windings for these types of wind turbine generators.
FIG. 1A and FIG. 1B, respectively, illustrate an end view and a side view for an exemplary slip ring assembly for conveying three phase ac power to the rotor of a wind turbine generator. The slip ring assembly 10 includes an insulated hub 15 with a cylindrical axial cavity 20 for receiving an end shaft of the wind turbine generator rotor. The slip ring assembly 10 includes, relative to the wind turbine generator, an outer end plate 25 and an inner end plate 30 with a ground ring 32. The outer end plate 25 and the inner end plate 30 include a cylindrical axial cavity to accommodate the insulated hub 15. The outer end plate 25, inner end plate 30 and insulated hub 15 are maintained in axial proximity by connection rods 35 threaded and bolted at the plate ends. Insulating pieces 40 surround the connection rods 35 and prevent contact of the connection rods 35 with the electric power in the slip rings. Three annular conductive contact rings 45 (one for each phase of power to the rotor windings) radially surround and are fixed to the slip ring assembly 10. The contact rings 45 provide a contact surface 50 for brushes (not shown) to transfer power to the slip ring assembly 10. The contact ring 45 for an individual phase is physically and electrically in contact with one or more conducting rods 55 for that phase. The conducting rods 55 extend outward through the outer end plate 25 and provide for connection to rotor cables (not shown) at the outer end of the rotor shaft (not shown) from the rotor windings (not shown). Insulating pieces 60 physically separate the contact rings 45 and isolate the conduction path through the conduction rods 55 for an individual phase of power to the rotor windings.
FIG. 2 illustrates electrical connections from the wind turbine generator rotor to the slip ring assembly. The rotor shaft 65 may include a rotor shaft end plate 70. An electrical connection plate 75 is mounted to threaded ends 80 of the conducting rods 55. The electrical connection plate 75 may be constructed of an insulating material to provide for physical connection with the conducting rods 55 for individual rotor phases while providing electrical isolation between the phases. A power cable 85 for each phase of the rotor windings extends from rotor shaft end plate 70. The power cable 85 may be connected to a conducting tab 90 on a phase connecting plate 95 to electrically connect the rotor winding (not shown) through the conducting rods 55 to the associated contact ring 45.
As part of the maintenance process of a wind turbine generator, slip ring assemblies are removed and refurbished to renew their operational capability. Removal of the rotor shaft from inside the hub is difficult since the rotor shaft is provided with heat-shrink fit within the hub. Even when the electrical connection plate is removed from the end of the slip ring assembly, there is no physical access available to pry the slip ring assembly from the generator side. Currently, 90% of these assemblies are damaged upon their removal, as a controlled, non-destructive method is not available. Many of the slip rings, which are damaged during removal for refurbishment, are scrapped because they are beyond repair. The slip rings are damaged by an inefficient removal method as a consistent and repeatable approach has not been employed. This results in increased material cost that is magnified by the hundreds of wind turbines in which this issue occurs. The current slip ring removal method involves hitting the assembly with a hammer. Such blows frequently strike the contact surface resulting in surface damage or strike the porcelain insulators, cracking or totally destroying them. While effective in removing the slip ring assembly, the resulting level of damage is unacceptable. This method is unreliable and also increases risk of personal injury.
Further, difficulty is encountered in removing the slip ring assembly because the wind turbine generator is mounted in the nacelle atop the wind turbine tower, often hundreds of feet above the ground.
Accordingly, there is a need to provide an apparatus and method for efficiently removing slip rings from the rotors for rotating electrical machinery without damaging the slip ring assembly.